The full-closure position of a throttle valve for adjusting an amount of intake air flowing into each cylinder of an internal combustion engine is set such that the throttle valve is inclined in an opening direction by a slight angle with respect to a position at which the throttle valve is perpendicular to an axis of an intake passage (i.e., such that the throttle valve does not interfere with an inner wall surface of the intake passage), because it is necessary to prevent the throttle valve from biting into (or being caught on) the inner wall surface within the intake passage (inner wall surface of a bore). Thus, a slight clearance is created between the throttle valve and the inner wall surface of the intake passage when the throttle valve is fully closed. The engine is idling when the throttle valve is fully closed. Therefore, the narrower the clearance created between the throttle valve and the inner wall surface of the intake passage during full closure of the throttle valve becomes, the lower the idling rotational speed of the engine can be achieved to reduce fuel consumption.
Thus, various modifications have been conventionally made to the butterfly valve-type throttle valve (such as the throttle valve) in order to make the clearance between the throttle valve and the inner wall surface of the intake passage as narrow as possible to reduce the amount of air leakage and hence reduce the idling rotational speed of the engine while preventing the throttle valve from biting into the inner wall surface of the intake passage when the throttle valve is at the full-closure position.
For example, according to disclosure in JP 09-329028 A, a wall portion of a valve holder is provided with a step portion, and a butterfly valve is brought into abutment on this step portion to ensure air-tightness during full-closure of the butterfly valve. According to disclosure in JP 08-277717 A, the sealability of respective intake passages and the like is secured in a so-called multi-arrayed variable intake valve or the like.
Patent Document 1: JP 09-329028 A
Patent Document 2: JP 08-277717 A
However, the related art has a problem in that the step portion of the valve holder may cause a resistance against the admission of intake air and the performance of the engine deteriorates as a result. Also, the clearance between a throttle valve according to the latter art and the inner wall surface of the intake passage cannot be reduced beyond a certain limit.
In particular, in the above-mentioned multi-arrayed variable intake valve, which is composed of a plurality of valve bodies fixed to a single valve shaft, the clearances between the respective valve bodies and the inner wall surfaces of the respective intake passages are liable to disperse when each of the valve bodies is at a full-closure position, owing to mounting errors of the respective valve bodies on the valve shaft, working errors of relevant components, or the like. It is extremely difficult to set those clearances equal to one another and narrower.
Thus, there is a need in the art to provide a butterfly valve-type throttle valve (a so-called butterfly valve) suitably employable as the above-mentioned multi-arrayed variable intake valve and makes it possible to set the clearance between the throttle valve and an inner wall surface of an intake passage narrower than before to reduce the leakage amount of inflow air while being prevented from biting into the inner wall surface of the intake passage mainly when the throttle valve is at a full-closure position or in a low-opening degree range.